Activity coefficients and type of non-ideality in n-alkanes mixtures predicted from equations of state: the role of the third parameter

QUINZIO, MARTINA JULIETA ; RODRIGUEZ REARTES, SABRINA BELÉN; CISMONDI, MARTÍN

Campinas

Conferencia; XII Iberoamerican Conference on Phase Equilibria and Fluid Properties for Process Design (Equifase 2022) / X Congresso Brasileiro de Termodinâmica Aplicada (CBTermo, 2022); 2022

UNICAMP

Two parameter equations of state (2P-EoS), as Soave-Redlich-Kwong and Peng-Robinson (PR) EoS, were proven in the literature to fail in describing the non-ideality of mixtures formed by n-alkanes with different chain length, which are considered as nearly athermal mixtures, i.e. with negligible mixing heat. Such models predict positive deviations for the activity coefficients at infinite dilution (γinf), which do not correspond with experimental behavior.In particular, in a previous work of our group it was shown that for binary systems formed by n-alkanes, PR with null interaction parameters predicts positive deviations from ideality, which increase with the asymmetry of the system, but experimental measurements evidence the opposite. In turn, when using either the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) EoS or the Generalized Redlich-Kwong-Peng-Robinson (RKPR) EoS with a recent parameterization for n-alkanes, both models qualitatively predict appropriate behaviors, being in general RKPR predictions closer to reported experimental data. It was concluded that the cubic nature of PR is not the reason for the incorrect trend observed for the predicted γinf, since the RKPR with the same repulsive term does not suffer from this limitation. Nor is it necessary to resort to more complex mixing rules, but with a third parameter (δ1 in RKPR) it is possible to capture the adequate evolution of the behavior of γinf in a family of compounds such as of n-alkanes, and give flexibility to the model. RKPR predictions are comparable and in many cases superior to those obtained with a SAFT-type equation, which involves greater mathematical complexity.In a more recent work by another research group, a scientific debate on these issues was opened. It was argued that using a null attractive interaction parameter (kij) condemns the PR equation to predict positive deviations from ideality and to fail in the prediction of systems that exhibit the opposite behavior. They proposed the use of temperature dependent kij (kij=f(T)) and showed that, in this way, PR could achieve a highly accurate prediction of γinf of n-hexane infinitely diluted in heavy n-alkanes. However, they recognize the superiority of RKPR, since it presents greater precision, especially when the asymmetry of the system increases. In the present work it is shown that PR with kij=f(T) improves the equilibrium prediction in the low pressure region, but it significantly underestimates the critical pressure (Pc) of the system. This effect is more evident the higher the asymmetry of the system and at higher temperatures. In the case of PC-SAFT with null kij, it overestimates the Pc, while RKPR with null kij achieves the best equilibrium prediction at both low and high pressures, and has the smallest deviation in the predicted Pc.On the other hand, the predictions of the γinf for heavy n-alkanes infinitely diluted in a light n-alkane (as n-hexane) show that PR with kij=f(T) manages to qualitatively capture the correct behavior, but present much greater deviations from the experimental data than the predictions obtained with PC-SAFT and RKPR, both with null kij, RKPR being the most accurate.